I. Field of the Invention
This invention relates generally to a device for converting organic material to a combustible gas, and more particularly to the particular design of a small scale gasifier which successfully obviates many of the problems inherent in prior art gasifier equipment.
II. Discussion of the Prior Art
Gasification, as used herein, refers to the production of combustible gases from solid organic material by the application of heat, i.e., pyrolysis. Gasifiers of the general type involved here have been around for over 200 years. During the Industrial Revolution, large quantities of coal were being coked prior to its use in smelting operations. The gas driven off during the coking process was combustible and was used for gas lighting during the early 1800's. Subsequently, gasifiers were designed for use with internal combustion engines. During World War I, with the blockading of oil imports, the German military utilized bolt-on gasifiers as a fuel source for motor vehicles.
Combustion, for purposes of gasification, can be defined as the chemical reaction between oxygen and an organic fuel, i.e., a fuel in which the element carbon is in its chemical makeup. During combustion, the oxygen chemically combines with the fuel to produce new chemical compounds and it is found that the rate of the reaction is dependent on many factors other than the chemical makeup of the fuel itself. For example, the amount of oxygen reaching the fuel has a great effect on combustion rate as does the amount of heat applied to the fuel to liberate the gases necessary for combustion to take place. Another factor is the physical characteristics of the fuel, i.e., its shape and total surface area exposed to oxygen.
Efficient gasification is also dependent upon the manner in which heat liberated during combustion of the fuel is absorbed by yet uncombusted material. Because heat rises, it follows that uncombusted fuel should be placed above the point where combustion is already underway.
Another variable which will alter or affect the properties of combustion lies in the manner in which the oxidizer is introduced to the fuel. The oxidizer, which is usually air, can be brought to the combustion zone from three main directions, i.e., from below, from above or from the sides. Each direction of air flow is found to exhibit its own particular advantages and disadvantages. The natural flow of air is from below. This is because the gases and smoke created by the fire are hotter than the surrounding atmosphere, and, therefore, lighter. This causes them to rise through the combustion zone which, in turn, draws more air in at the bottom to replace it. The advantage of this natural convection air supply is that it is self-feeding and requires no outside impetus to air movement such as a blower. The drawbacks of the natural flow are that tars and other uncombusted by-products are carried off by the exiting gases and smoke creating pollution problems. The tars and by-products also tend to re-condense on the walls of the gasifier unit requiring periodic shutdown for cleanup. Furthermore, the tar substances passing upward through the fuel mass tends to condense out creating a sticky residue on the fuel, inhibiting its ability to flow.
In an attempt to alleviate or eliminate the tar production problem, a number of prior art gasifiers have been designed which deliver combustion air from the sides. In the single side delivery system, the combustion zone takes on the configuration of an elongated ovoid. This proved to be rather counter-productive, in that the cross-sectional configuration of the containment vessel is most often circular. In practice, it means that some uncombusted feed stock will simply move past the sides of the combustion zone and fall into the ash pit. Later, modifications were made to bring combustion air in from a number of discrete directions which gives rise to a combustion zone of overlapping ovoids. For example, if air inlets are positioned 60.degree. apart around the periphery of the combustion zone, each air inlet will only carry 16.6% of the air carried by a single inlet. This leads to radically decreased ovoids which barely overlap. The end-result is, again, poor combustion characteristics.
The side delivery designs permit combustion gases to exit through the uncombusted feed stock, which carried off the tars from the partial pyrolysis. It has been determined that tar production ceases above 700.degree. C. Any tars that are produced above that temperature are quickly decomposed into simpler chemical constituents. Thus, tar that is produced in feed-stocks below the 70.degree. C. limit may be broken down at will by the simple expedient of heating the tar above the 70.degree. C. temperature. Perhaps the simplest way of accomplishing this is to bring combustion air into the fire, or gasifier, from the top. The combustion air carries any tar products and vapors along with it directly into the hot combustion zone of the gasifier. The exit for the combustion gases is through the combustion zone and out the bottom of the fire.
Another problem extant in prior art gasifier designs involves "bridging" where the incoming feed stock builds up in the combustion zone and does not naturally flow as combustion takes place. The practice in dealing with the fuel bridging problem has been to provide mechanical agitators for stirring up the organic material feed stock and breaking up the bridged fuel so that it can continue to flow into the combustion zone. Such mechanical devices need attention and are also subject to frequent repair and replacement.
A successful gasifier system should exhibit the following characteristics:
1. Zero bridging; PA1 2. Minimal maintenance; PA1 3. Minimal down-time for cleaning and/or ash removal; and PA1 4. Usable generated gas, i.e., no tar, no condensate, no particulate matter, no obnoxious emissions.
The system of the present invention possesses all of the above attributes. It is capable of handling a wide range of feed stocks in terms of types and sizes and requires no operator in attendance. The system is capable of running for prolonged periods without the need for periodic shutdowns. It produces no tar, condensates, hydrocarbons or obnoxious emissions and satisfies all EPA guidelines. The invention presents no bridging or feeding problems and drastically reduces clean-up and ash handling. The gasifier itself is totally self-cleaning and all ash generated is deposited in a receptacle without recourse to augers or mechanical devices of any kind. The system of the present invention does not require any down-stream gas clean-up apparatus in that the generated gases are ready to use as they are generated.